Narrow Results

In this paper, we discuss the concept of Symmetric Teleparallel Gravity and its extensions, mainly focusing on $f(Q,T)$ modified gravity theories. These theories explore the replacement of the Levi–Civita connection in General Relativity with a general affine connection in flat spacetime, leading to the formulation of gravity based on non-metricity. We highlight the advantages of second-order field equations in $f(Q)$ and $f(Q,T)$ theories compared to higher-order equations in modified $f(R)$ gravity. The inclusion of bulk viscosity in the model offers insights into the late-time behavior of the universe. In this regard, we study the spatially homogeneous and anisotropic universe with viscous fluid in the framework of the symmetric teleparallel gravity $f(Q,T)$. We solve the field equations and obtain some cosmological parameters, including the deceleration parameter, equation of state parameter, Bulk viscous pressure and scale factor. In the following, we analyze the behavior of the cosmological parameters of the model with some figures and adjust our model parameters with recent data.

In this paper, the 1σ confidence level of transition redshift and present deceleration parameter are studied using modified Chaplygin gas (MCG) model from recent observations: the 182 Gold SNe Ia, three-year WMAP, SDSS baryon acoustic peak and X-ray gas mass fraction in clusters. Concretely, the best fit values of the transition redshift and present deceleration parameter with the 1σ confidence level are and , which indicate the best fit values of z_T and q₀ for MCG case are surprisingly close to the central values given by Refs. 20 and 21. Furthermore, we plot the evolution of cosmological quantities from MCG model by using the best-fit parameters given by combining constraint, and compare them with ΛCDM model.

In this paper, a parametrized deceleration parameter q(a) = q₀+q₁(1-a) is constrained by using the current cosmic observational data from type Ia Supernova (Sne Ia) and Cosmic Microwave Background Radiation (CMB). When the CMB dataset is added as a strong constraint, it is found that the 1σ error is largely reduced. The values of transition redshift z_T from decelerated expansion to accelerated expansion and current deceleration parameter q₀ are larger than that obtained from the case where Sne Ia dataset is used alone. With comparison to the case of Sne Ia 182 dataset used,¹⁵ it is found that the value of transition redshift is smaller than that in Sne 192 dataset case. This is the so-called dataset dependence.

The present-day accelerated expansion of the universe is naturally addressed within the Brans–Dicke theory just by using holographic dark energy model with inverse of Hubble scale as IR cutoff and power law temporal behavior of scale factor. It is also concluded that if the universe continues to expand, then one day it might be completely filled with dark energy.

We apply the type Ia supernovae union dataset and the baryon acoustic oscillations data at z = 0.2 and z = 0.35 to constrain variable Chaplygin gas (VCG) model as the unification of dark matter and dark energy. It is shown that the confidence levels for VCG model parameters are . And it indicates that the values of transition redshift and current deceleration parameter are: . In addition, we plot the evolution trajectory of the VCG model in the statefinder parameter r–s plane and show the discrimination between this scenario and other dark energy models.

The paper deals with a spatially homogeneous and anisotropic universe filled with perfect fluid and dark energy components. The two sources are assumed to interact minimally together with a special law of variation for the average Hubble's parameter in order to solve the Einstein's field equations. The law yields two explicit forms of the scale factor governing the Bianchi-V spacetime and constant values of deceleration parameter. The role of dark energy with variable equation of state parameter has been studied in detail in the evolution of Bianchi-V universe. It has been found that dark energy dominates the universe at the present epoch, which is consistent with the observations. The universe achieves flatness after the dominance of dark energy. The physical behavior of the universe has been discussed in detail.

We present a theory of weak gravity parametrized by a fundamental frequency ${\omega }_0=\sqrt{1-q}H$ of the cosmological horizon, where $H$ and $q$ denote the Hubble and, respectively, deceleration parameter. It predicts (i) a $C^0$ onset to weak gravity across accelerations $\alpha =a_{\mathrm{\text{dS}}}$ in galaxy rotation curves, where $a_{\mathrm{\text{dS}}}=cH$ denotes the de Sitter acceleration with velocity of light $c$, and (ii) fast evolution $Q(z)=dq(z)/dz$ of the deceleration parameter by $\mathrm{\Lambda }={\omega }_0^2$ satisfying $Q_0>2.5$, $Q_0=Q(0)$, distinct from $Q_0\lesssim 1$ in $\mathrm{\Lambda }$CDM. The first is identified in the high resolution data of Lelli et al. (2017), the second in the heterogeneous data on $H(z)$ over $0<z<2$. A model-independent cubic fit in the second rules out $\mathrm{\Lambda }$CDM by $4.35\sigma$ and obtains $H_0=74.0\pm 2.2{\mathrm{\text{km s}}}^{-1}{\mathrm{\text{Mpc}}}^{-1}$ consistent with Riess et al. [Astrophys. J.826, 56 (2016)]. Comments on possible experimental tests by the LISA Pathfinder are included.

In this paper, we have proposed a generalized parametrization for the deceleration parameter q in order to study the evolutionary history of the universe. We have shown that the proposed model can reproduce three well known q-parametrized models for some specific values of the model parameter $\alpha$. We have used the latest compilation of the Hubble parameter measurements obtained from the cosmic chronometer (CC) method (in combination with the local value of the Hubble constant $H_0$) and the Type Ia supernova (SNIa) data to place constraints on the parameters of the model for different values of $\alpha$. We have found that the resulting constraints on the deceleration parameter and the dark energy equation of state support the $\mathrm{\Lambda }$CDM model within $1\sigma$ confidence level at the present epoch.

The magnetized strange quark matter (MSQM) solutions are obtained for a Marder type universe using constant deceleration parameter. The exact solutions of field equations are obtained for f(R,T) = R + 2f(T) model given by [T. Harko et al., Phys. Rev. D84, 024020 (2011)] with cosmological term. Also, we have obtained General Relativity (GR) solutions for MSQM distributions. For $t\to \infty$, we get the dark energy model, i.e. $p\to -B_c$, $\rho \to B_c$ and $\omega =-1$. However, for $t\to \infty$, we find the cosmological constant $\mathrm{\Lambda }$ as negative in f(R,T) theory and GR. These results agree with [S. Aygün et al., Astrophys. Space Sci.361, 380 (2016); C. Aktaş and S. Aygün, Chinese J. Phys.55, 71 (2017); P. K. Sahoo et al., Mod. Phys. Lett. A32, 1750105 (2017)] in f(R,T) theory. The physical consequences of our obtained models are discussed at the end.

In this paper, we reconstruct the late-time cosmological dynamics using a purely kinematic approach. In particular, considering a divergence-free parametrization for deceleration parameter $q$, we first derive the jerk parameter $j$ and then confront it with combination of various cosmological datasets. We use the most recent observational datasets consisting of the 1048 Pantheon Supernovae Ia data points in the redshift range $0.01<z<2.3$, the 51 data points of observational Hubble parameter (OHD) measurements in the redshift range $0.07\le z\le 2.36$, the Hubble constant $H_0$ (R19) and the CMB shift parameter measurements. We study the evolution of different cosmological quantities for the present model and compare it with the concordance $\mathrm{\Lambda }$CDM model. We find that only the combined Pantheon+OHD+R19 data shows good agreement with the $\mathrm{\Lambda }$CDM $(j=1)$ model within $1\sigma$ confidence region. We also find that our model successfully generates late time cosmic acceleration along with a decelerated expansion in the past.

This study investigates the dynamics of a spatially homogeneous and anisotropic LRS Bianchi type-I Universe with viscous fluid in the framework of $f(Q)$ symmetric teleparallel gravity. We assume a linear form for $f(Q)$ and introduce hypotheses regarding the relationship between the expansion and shear scalars, as well as the Hubble parameter and bulk viscous coefficient. The model is constrained using three observational datasets: the Hubble dataset (31 data points), the Pantheon SN dataset (1048 data points), and the BAO dataset (6 data points). The calculated cosmological parameters indicate expected behavior for matter-energy density and bulk viscous pressure, supporting the universe’s accelerating expansion. Diagnostic tests suggest that the model aligns with a $\mathrm{\Lambda }$CDM model in the far future and resides in the quintessence region. These findings are consistent with recent observational data and contribute to our understanding of cosmic evolution within the context of modified gravity and bulk viscosity.

This research focuses on exploring the dynamics of a cosmos model under the influence of Ricci dark energy within the framework of modified Lyra geometry. The modified field equations of Einstein for Lyra’s geometry are introduced, and specific solutions are obtained for a Big Rip cosmos scenario. Additionally, various physical and geometrical aspects of the cosmos are comprehensively investigated and discussed.

In this study, we explore the enigma of late-time cosmic acceleration in the framework of a general scalar field playing the role of dark energy. Our investigation yields an exact solution to Einstein’s field equations, achieved through a parametrization of the deceleration parameter $q(a)$ in quadratic form in the homogeneous and isotropic background geometry represented by Friedmann–Lemaître–Robertson–Walker (FLRW) spacetime. This parametrization unveils a seamless transition from a decelerated to an accelerated phase in the cosmic evolution. Furthermore, we leverage this parametrization to reconstruct cosmological parameters with respect to redshift z, elucidating their dependency on certain model parameters that govern the universe’s evolutionary trajectory. The model parameters are subsequently constrained using diverse observational datasets. We provide a comprehensive analysis of the evolution of geometrical and physical parameters, complete with the numerically constrained values. To gain deeper insights into the nature of dark energy, we employ Statefinder diagnostics, conducting a thorough examination. Our findings shed light on the intricate interplay between cosmic acceleration, quintessential dark energy and the parametrization of deceleration parameter, offering valuable insights into the fundamental dynamics of the universe.

We present a simple higher dimensional FRW-type of model where the acceleration is apparently caused by the presence of the extra dimensions. Assuming an ansatz in the form of the deceleration parameter, we get a class of solutions some of which shows the desirable feature of dimensional reduction as well as reasonably good physical properties of matter. Interestingly we do not have to invoke an extraneous scalar field or a cosmological constant to account for this acceleration. One argues that the terms containing the higher dimensional metric coefficients produces an extra negative pressure that apparently drives the inflation of the 4D space with an accelerating phase. It is further found that in line with the physical requirements our model admits of a decelerating phase in the early era along with an accelerating phase at present. Further the models asymptotically mimic a steady-state-type of universe although it starts from a big-bang-type of singularity. Correspondence to Wesson's induced matter theory is also briefly discussed and, in line with it, it is argued that the terms containing the higher dimensional metric coefficients apparently creates a negative pressure which drives the inflation of the 3-space with an accelerating phase.

A spatially homogeneous and anisotropic Bianchi type-I space–time has been studied within the framework of Lyra's geometry. Exact solutions of the Einstein's field equations have been obtained with a time dependent gauge function by using a special law of variation for Hubble's parameter that yields a constant value of deceleration parameter. It has been found that the solutions generalize the solutions obtained by Rahaman et al. [Astrophys. Space Sci.299, 211 (2005)] and are consistent with the recent observations of type Ia supernovae. A detailed study of physical and kinematical properties of the model has been carried out.

A five-dimensional FRW-type Kaluza–Klein cosmological model is taken to study the role of extra dimension in the expansion of the universe. Relation between scale factors corresponding to conventional four dimensions and the extra dimension has been established. Field equations are solved in order to find out the effect of pressure corresponding to these scale factors. Conditions for accelerated expansion are derived.

A general approach for investigating Bianchi-V cosmological models is introduced in a scalar-tensor theory of gravitation proposed by Saez and Ballester in which the law of variation for Hubble's parameter is taken into account. This variation for Hubble's parameter that yields a constant value of deceleration parameter is then utilized to solve the field equations governing anisotropic Bianchi-V space–time filled with perfect fluid. Two types of exact solutions that correspond to singular and nonsingular models are presented. Finally, we arrive to the conclusion that the universe decelerates for positive value of deceleration parameter whereas it accelerates for negative one. The behaviors of observationally important parameters are discussed in detail. Exact expressions for look-back time, luminosity distance and event horizon versus redshift are derived and their significance are discussed in detail. It has been observed that the solutions are compatible with the results of recent observations.

We study spatially homogeneous and anisotropic Bianchi-V perfect fluid cosmological model with variable G and Λ. Exact solutions to the field equations in the case of particle creation and in the absence of particle creation are obtained by assuming a constant deceleration parameter. The model has a big-bang-type singularity at the initial time t = 0. We find that a constant value of deceleration parameter is reasonable for a description of the present day universe. We also observe that the variable G does not necessarily imply particle creation. The behavior of observationally important parameters such as the expansion scalar, mean anisotropic parameter and shear scalar are discussed. It is observed that the solutions are compatible with the results of recent observations.

In this paper we investigate an observable universe in Bianchi type V space–time by taking into account the cosmological constant as the source of energy. We have performed an ${\chi }^2$ test to obtain the best fit values of the model parameters of the universe in the derived model. We have used two types of data sets, viz: (i) 31 values of the Hubble parameter and (ii) the 1048 Pantheon data set of various supernovae distance moduli and apparent magnitudes. From both the data sets, we have estimated the current values of the Hubble constant, density parameters ${({\mathrm{\Omega }}_m)}_0$ and ${({\mathrm{\Omega }}_{\mathrm{\Lambda }})}_0$. The present value of deceleration parameter of the universe in derived model is obtained as $q_0=-0.59_{-0.03}^{+0.04}$ and $-0.59_{-0.03}^{+0.02}$ in accordance with $H(z)$ and Pantheon data, respectively. Also we observe that there is a signature flipping in the sign of deceleration parameter from positive to negative and a transition redshift exists. Thus, the universe in the derived model represents a transitioning universe which is in an accelerated phase of expansion at present epoch. We have estimated the current age of the universe $(t_0)$ and present value of the jerk parameter $(j_0)$. Our obtained values of $t_0$ and $j_0$ are in good agreement with their values estimated by Planck collaborations and WMAP observations.

In this paper, we have proposed a model of accelerating universe with binary mixture of bulk viscous fluid and dark energy (DE) and probed the model parameters: present values of Hubble’s constant $H_0$, equation of state paper of DE ${\omega }_{\mathrm{\text{de}}}$ and density parameter of DE ${({\mathrm{\Omega }}_{\mathrm{\text{de}}})}_0$ with recent observational $H(z)$ data (OHD) as well as joint Pantheon compilation of SN Ia data and OHD. Using cosmic chronometric technique, we obtain $H_0=69.80\pm 1.64{\mathrm{\text{km s}}}^{-1}{\mathrm{\text{Mpc}}}^{-1}$ and $70.0258\pm 1.72{\mathrm{\text{km s}}}^{-1}{\mathrm{\text{Mpc}}}^{-1}$ by restricting our derived model with recent OHD and joint Pantheon compilation SN Ia data and OHD, respectively. The present age of the universe in derived model is estimated as $t_0=13.82\pm 0.33\mathrm{\text{Gyr}}$. Also, we observe that derived model represents a model of transitioning universe with transition redshift $z_t=0.7286$. We have constrained the present value of jerk parameter as $j_0=0.969\pm 0.0075$ with joint OHD and Pantheon data. From this analysis, we observed that the model of the universe, presented in this paper, shows a marginal departure from $\mathrm{\Lambda }$CDM model.